I don't see any problem with storing your wallet files on an external hard drive, assuming you take the proper precautions when securing the drive itself. E.g. You can encrypt the files within a password protected archive, or simply use a password protected drive such as the following.

https://www.amazon.com/lepin-Military-AES-CBC-Encrypted-Advanced/dp/B06X6NP89Y/ref=sr_1_11?ie=UTF8&qid=1516345200&sr=8-11&keywords=password+thumb+drive

Of course, you could simply use a trezor or ledger hardware wallet.